Dualism and Interaction

Descartes’s Real Distinction argument in the Sixth Meditation, if successful, shows that the are two distinct and independent kinds of substantial being—viz. mental and material. This position, and Descartes’s argument for it, raises all sorts of questions. But there is a further claim that Descartes’s commits himself to that is especially puzzling. Descartes clearly thinks that in the case of human beings mental and material substance is closely linked. He says,

I am present in my body not merely in the way a sailor is present in a ship, but that I am most tightly joined and, so to speak, commingled with it, so much so that I and the body constitute one single thing. (7:81)

The connection between mind and body is necessary to explain both the sensory experience of various conditions of the embodied self (e.g. hunger, joy, disgust, exhaustion, etc.), and the intentional movements of the body. The claim thus is that one’s body (i.e. that material substance “tightly joined” to the mental substance refereed to by the “I” in “I think”) has at least some of its properties (its “modes”) because of what the mind does, and vice versa.

In this manner Descartes seems committed to a position where mental and material substance causally interact, at least in one direction, from the mind to the body. Call this position “interactionism”. How are we to understand the causal interaction of mental and material substance?

Descartes’s exchange of letters with Elisabeth, Princess Palatine of Bohemia (1618–1680) clearly sets out both the problems inherent in the interactionist position and the difficult nature of his replies. Let’s look at the central points brought out by their exchange.

Causal Interaction of Mind and Body

The Problem of Movement

First, Elisabeth asks how could the mind move the body and vice versa?

it seems that all determination of movement happens through the impulsion of the thing moved, by the manner in which it is pushed by that which moves it, or else by the particular qualities and shape of the surface of the latter. Physical contact is required for the first two conditions, extension for the third. (3:661)

Elisabeth’s point is straightforward. Descartes’s position on the human being as the “commingling” of mind and body, along with the explanation of intentional bodily movement by the mind, requires that he be able to explain how the mind could move the body. But our conception of how such an explanation could proceed presumes one body acting on another because all bodily movement (so it seems) requires impulsion or force applied to the surface of a body. But the application of such force seems to require both physical contact and extension, and these are two properties that are lacked by mental substance.

Descartes replies to this objection by claiming that there are three “primitive notions” (i.e. mind, body, and mind-body unity). These are “primitive” in the sense that we cannot expect them to be explicable in any other way than through themselves. He says,

Although the use of the senses has given us notions of extension, of shapes, and of movements that are much more familiar than the others, the principal cause of our errors lies in our ordinarily wanting to use these notions to explain those things to which they do not pertain. (3:666)

Descartes also contends that we confuse one kind of causation with another in many of the explanations we give of intentional action, or of the condition of one’s body causing a change in one’s mind (e.g. of pain).

I believe that we have heretofore confused the notion of the power with which the soul acts on the body with the power with which one body acts on another; and that we have attributed the one and the other not to the soul, for we did not yet know it, but to diverse qualities of bodies, such as heaviness, heat, and others, which we have imagined to be real, that is to say, to have an existence distinct from that of body, and by consequence, to be substances, even though we have named them qualities. (3:667)

We can thus summarize Descartes’s position by saying that we have within us, or innately, three basic ideas, that of mind, that of body, and that of their union. Each is separate, each is distinct, and each has its own domain of application; each is per se intelligible, and cannot be explained in terms of other primitive notions.1

This is not an incoherent position. It does, however, put significant pressure on Descartes’s dualism. For it would seem to entail that out conceptions of the essences of body and mind does not form the basis of the third “primitive notion” that is the mind-body union. If they did then this would be to say that the mind-body union could be explained or understood in terms of the ideas of mind and body, which is what Elisabeth is trying to do, and which she is quite understandably puzzled about. But this would mean that the human being is a kind of being whose properties (or modes) do not all follow either from thinking or from extension. But a substance is a being whose modes all are derived from its basic attribute. Hence the human being, in not deriving all of its modes from either thought or extension, would seem to be a different kind of being from that of mind and body. Thus, by positing a “primitive notion” of the mind-body union Descartes appears to undermine his own dualist position in favor of “trialism” or the position that there are three kinds of substance: mind, body, and the mind-body (i.e. the human being).2

The Immateriality of Mind

A further problem Elisabeth raises for Descartes’s position concerns his characterization of mental substance as immaterial (i.e. as not material). As Elisabeth points out, this does not tell us anything positive about the nature of mind, but only that it is, in this respect the “negation of matter” (3:684-5). And this again leads to an obvious problem. How could something that is characterized as the negation of matter have any positive interaction with it, especially when all interaction between matter depends on its materiality as extension in space? As Elisabeth goes on to say,

it would be easier for me to concede matter and extension to the soul than to concede the capacity to move a body and to be moved by it to an immaterial thing (3:685)

Descartes goes on to make three points, none of which are particularly convincing. First, he argues that we cannot conceive clearly the mind-body union (3:691-2). He says,

those things which pertain to the union of the soul and the body are known only obscurely by the understanding alone, or even by the understanding aided by the imagination; but they are known very clearly by the senses.

Again though, the problem is that the mind-body union should be intelligible through our understanding of mind and body. If it is not this seems to indicate that it is some third kind of thing.

Descartes does try and counter Elisabeth’s claim that it is easier to think of the mind as extended than it is to think of an unextended thing causing an extended thing to move. He says

since your Highness notices that it is easier to attribute matter and extension to the soul than to attribute to it the capacity to move a body and to be moved by one without having matter, I beg her to feel free to attribute this matter and this extension to the soul, for to do so is to do nothing but conceive it as united with the body. (3:694)

Here Descartes attempts to explain away the comparative ease of thinking of the mind as extended as just a feature of thinking about the mind’s activity of the mind-body union.

Finally, Descartes says, in what can only seem to be a kind of patronizing tone, that metaphysics is not the kind of thing one should spend significant amounts of time thinking about.

though I believe it is very necessary to have understood well once in one’s life the principles of metaphysics, since it is these that give us knowledge of God and of our soul, I also believe that it would be very harmful to occupy one’s understanding often in meditating on them. For in doing so, it could not attend so well to the functions of the imagination and the senses. The best is to content oneself in retaining in one’s memory and in one’s belief the conclusions that one has at one time drawn from such meditation, and then to employ the rest of the time one has for study in those thoughts where the understanding acts with imagination or the senses. (3:695, see also 3:692-3)

Unfortunately this looks more like an attempt to shut down conversation about a matter for which Descartes has little convincing to say, than it does an honest attempt to convey his position. After all, it seems that Elisabeth (and the reader!) is trying to “understand well” the dualist and interactionist position for which Descartes has advocated.


Cottingham, John. 2008. “Cartesian Trialism.” In Cartesian Reflections: Essays on Descartes’s Philosophy, edited by , 173–87. Oxford: Oxford University Press.

Garber, Daniel. 1983. “Understanding Interaction: What Descartes Should Have Told Elisabeth.” The Southern Journal of Philosophy 21 (1):15–32.

  1. See (Garber 1983). ↩︎

  2. See (Cottingham 2008) for discussion and defense of trialism ↩︎

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